I do not have all these rifles on hand to try but in general a 742 mag wil work in a 7400 or a 750 ?? but how about a 760 
granted a long action (30-06) mag and a short action mag will not be interchangable but will they just between models . Sometimes you find one at a good price but not the other ?
thanks ,
dscp
In one sentence, Remington 740, 742, 7400, Model 4, 760, Model 6 and 7600 magazines will all interchange, short or long action, doesn't matter since all the guns are built on a long based action. I can't say anything about a 750, as I have never even seen one.
Since the guns are all built on a long based action, the difference in long versus short action magazine is within the magazine itself, the short action magazine has plastic spacers in it to make it smaller. That doesn't mean that a short action magazine(308 for example) won't fit into a rifle chambered for a 30-06, it will, but you're not going to be able to load 30-06 rounds into that magazine.
As Rangr44 points out the autos have a 'bolt hold open' feature so the action stays open when the last round is ejected after firing, but thats not an issue as far as function in the gun, ie, if you used a 760 magazine in a 742 of the same caliber, it will work, however it will not keep the action open on the last round. I do not know of an aftermarket magazine that has that feature.
I have a pair of aftermarket Triple K magazines according to the box, they fit a Remington 740, and no bolt open feature for short action. They fit and function perfectly in my 7400 in .308(thats what I purchased them for), however, the bolt doesn't stay open on the last shot since they don't have the 'bolt hold open feature'. They will also snap right into my 760, however, the 760 is a 30-06 so as stated above, because of the internal design, I can't load 30-06 cartridges into them. However, the point is, if the 7400 and 760 were the same caliber, I could use the magazines in either.
Hope this helps a little.
